The present invention relates generally to conditioning elements for telecommunications transmission facilities and, more particularly, to a transmit path limiting circuit for a caller identification device. The digital telecommunications transmission facilities include a central office which may transmit digital data signals over digital transmission lines, remote terminal channel units, and customer premises. Typically, the signals are sent over transmission lines differentially on two pairs of cables.
The Bell telephone system in the United States, for example, has widely utilized digital "D" multiplexing code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 transmission system. One pair of cables is provided for each direction of transmission.
The central office provides a digital data signal transmitted on two pairs of cables. The digital signals are then transmitted to the remote terminal channel unit. The digital data is then converted to an analog signal and provided on a two-wire ("tip-ring" conductor pair) to the customer's premises. The remote terminal unit includes a hybrid circuit that interfaces a two-wire transmit signal and a two-wire receive signal (a four-wire circuit) to the tip-ring pair.
The customer's premises may include a caller identification box which interprets information sent by the central office regarding the party originating the call. Such information is normally found in the signal provided to the customer's telephone after the first ring of a telephone being initiated.
Some equipment, such as the caller identification box, may not provide a substantial load on the circuit. The analog signal provided to the telephone and caller identification box may be looped back along the tip ring conductor pair to the remote terminal channel unit. The reflected signal may then be processed by the transmission line system, much in the same way that any other analog telephone signal coming from the telephone would be processed.
The reflected signal may then be retransmitted back along the transmission facilities to, for example, the central office. The central office may then loop back the signal toward the customer's premises, treating it as a significant digital data signal, even though it is, in fact, only a reflection or "echo," of the signal sent to the customer premises. Such reflected signals impose noise on the transmission lines, which may interfere with a caller identification box's ability to interpret the telephone number data on the telephone lines. Such data is typically sent immediately following the first ring of a call.